1. Field of the Invention
This invention relates, in general, to paper or cardboard machines and, in particular, to a new and useful method of improving the drying characteristics of the cylinder drying section of such machine.
2. Description of the Prior Art
A very important drawback in cylinder driers for paper and cardboard is the tendency of the paper of cardboard web to dry more at the edges than at the middle of the web. This uneven transverse moisture profile remains also in the finished product and causes considerable disadvantages in the subsequent treatment of the product.
The fast drying tendency of the border area is especially pronounced in high speed machines for drying thick webs wherein the temperature of the cylinder is kept high in order to achieve a sufficient drying of the web. One reason for this inconvenience is the hot border area of the cylinder will not be covered by a cooling paper web as under normal circumstances the width of the paper web does not extend over the whole length of the cylinder but uncovered narrow free edge strips remain at the ends of the cylinder. In this case the free edge strip will act as a heating rib promoting the drying of the paper at its border areas. The drying effect at the border area is further enhanced by the fact that the heating steam is often fed into the drying cylinder close to end thereof.
In order to eliminate the problems connected with excess drying of the border area, solutions have been proposed as regards the drying cylinder as well as the drying process. The solutions as regards the drying cylinder have aimed at changing the surface temperature of the cylinder so as to cool the border parts with respect to the middle part of the cylinder surface. In order to achieve this an insulating layer has been built inside the cylinder jacket covering the border areas. In some cases the cylinder jacket has been made thicker at its borders as compared to the middle part. One solution consists in boring cavities on the inside of the cylinder jacket into which are inserted bodies of a material having better thermal conductivity characteristics than the jacket material. If a greater number of these well conducting bodies are inserted at the centre of the cylinder than at the borders, this naturally leads to a temperature rise at the center part of the jacket as compared to the temperature at the borders, which affects the moisture profile advantageously. Further a cylinder construction has been proposed wherein the cylinder is divided longitudinally into separate heating compartments having separate inlets for the heating steam and corresponding condensate outlets.
By means of all the above mentioned cylinder construction solutions naturally the transverse moisture profile may be adjusted but it has not been possible to correct the profile completely except in some rare exceptional cases. It is further to be observed that the cylinders in the proposed solutions require fairly expensive special constructional arrangements which irrespective of their high costs do not function completely satisfactorily.
One possiblility exhibiting several different alternatives resides in attempting to affect the drying process from the outside of the cylinder and to dry the paper or the board web evenly in the crosswise direction. Solutions of this kind are offered especially by the different blowing and conducting systems for the drying air, by means of which the drying air is distributed differently towards different areas of the web, or by means of which moist air is withdrawn from the spaces between the cylinders. These solutions always require extremely expensive additional equipment the performance of which by no means is satisfactory the moisture profile still remaining uneven, i.e. the web tends to dry more at its edges than at its middle.